1. Field of the Invention
The present invention relates to a backlight unit and a Liquid-Crystal Display (LCD) device and more particularly, to a backlight unit structured to emit light intermittently from the light source, and a LCD device equipped with the backlight unit.
2. Description of the Related Art
In recent years, the LCD device has been extensively used as a high-resolution display device. The LCD device comprises a substrate on which switching elements such as Thin-Film Transistors (TFTs) are formed (which will be termed the “TFT substrate” below), another substrate on which a color filter and a black matrix are formed (which will be termed the “opposite substrates” below), and a liquid crystal layer sandwiched between the TFT substrate and the opposite substrate. An electric field is applied across the electrodes formed on the TFT substrate and those formed on the opposite substrate, or across the electrodes formed on the TFT substrate and the other electrodes formed on the said TFT substrate, thereby changing the alignment direction of the liquid crystal molecules in the liquid crystal layer. In this way, the amount of the transmitted light is controlled in each pixel to display desired images. The structure formed by the TFT substrate, the opposite substrate, and the liquid-crystal layer placed therebetween is termed the LCD panel.
With the transmissive type LCD device, a backlight unit is mounted on the back of the LCD panel, where the light emitted from the backlight unit is irradiated to the LCD panel from its back. As the light source for the backlight unit, conventionally, Cold Cathode Fluorescent Tubes (CCFLs) were being generally used. However, recently, the use of Light-Emitting Diodes (LEDs) has been increasing. In this case, many red, green and blue LEDs are used in combination to perform color mixture optically, thereby generating white light. The white light thus generated is irradiated to the LCD panel.
With the LCD devices comprising the backlight unit into which LEDs are incorporated, white light with a constant chromaticity needs to be generated at all times. Therefore, the driving currents supplied respectively to the red, green, and blue LEDs are feedback-controlled in such a way that the red, green, and blue lights from the respective LEDs are always mixed at a constant ratio, while the quantity of the light of each color is detected. If this feedback control is performed at high speed, the change of the chromaticity will be recognized by the user and therefore, it is performed at comparatively low speed. For this reason, the chromaticity is not adjusted as desired and as a result, there arises a phenomenon that some colors different from white are displayed on the screen at the time immediately after the turn on of the LCD device and thereafter, these non-white colors will be gradually turned to white. To suppress this phenomenon, conventionally, the driving currents for the red, green, and blue LEDs are respectively set at their predetermined initial values at the time immediately after the turn on of the LCD device.
On the other hand, it is general that the temperature of a LED at the time immediately after the turn on and that in the steady-state operation thereof are widely different, and that the optical characteristics of a LED varies greatly dependent on the temperature. Therefore, a LED has a disadvantage that it takes a long time from the time immediately after the turn on to the time when the emitted white light is converged to a predetermined chromaticity. A LCD device that has overcome the said disadvantage is disclosed in the Patent Document 1 (the Japanese Non-Examined Patent Publication No. 2006-171693) published in 2006. This LCD device is shown in FIG. 1.
FIG. 1 is a functional block diagram showing the structure of the prior-art LCD device 100 disclosed in the Patent Document 1.
The prior-art LCD device 100 of FIG. 1 comprises a transmissive type color LCD panel 103 and a backlight unit 112. The LCD panel 103 is driven by a X driver circuit 106 and a Y driver circuit 107. The backlight unit 112 comprises a light source 110 using red, green, and blue LEDs, and a wavelength selection filter 111, where the LEDs of each color are driven by a driver section 113. The temperatures of the LEDs are detected by temperature sensors 114. The chromaticity of the white light emitted from the LEDs is detected by chromaticity sensor 115 serving as a photosensor.
When an image signal is inputted into the LCD device 100 by way of an input terminal 101, the image signal is subjected to a predetermined signal process such as the chroma process in a RGB process controller 102 and then, it is converted to RGB separated signals which are suitable for driving the LCD panel 103. The RGB separated signals thus generated are supplied to a controller section 104 and at the same time, they are supplied to the X driver circuit 106 too by way of an image memory 105. The controller section 104 controls the X and Y driver circuits 106 and 107 with predetermined timing that is in accordance with the RGB separated signals received, and drives the LCD panel 103 using the RGB separated signals supplied to the X and Y driver circuits 106 and 107 by way of the image memory 105. In this way, images are displayed on the screen of the LCD panel 103 according to the RGB separated signals.
The driver section 113 supplies the predetermined currents to the respective LEDs of the light source 110 to drive them. At the same time as this, the driver section 113 feedback-controls the electric current quantities for the LEDs of each color based on the detected value of the chromaticity sensor 115, thereby adjusting the white light to the predetermined chromaticity value. Moreover, the driver section 113 reads the initial current values for the LEDs of each color at the turn on from a nonvolatile memory 116, compensates the initial current values thus read corresponding to the detected temperature values of the temperature sensors 114, and activates the LEDs of each color using the initial current values thus compensated.
With the prior-art LCD device 100 shown in FIG. 1, because the above-described structure is provided, the LEDs of each color can be activated to have their predetermined chromaticities from the time immediately after the turn on regardless of the temperatures of the LEDs at the turn on. Accordingly, the necessary time from the time immediately after the turn on to the time the emitted lights from the LEDs are converged to their predetermined chromaticities (i.e., from the time immediately after the turn on to the time white light with the predetermined chromaticity is generated stably) can be shortened. (See FIG. 4 and Abstract of the Patent Document 1.)
Moreover, a LCD device has a disadvantage that if a moving or time-varying image (or animation) is displayed, the contours of the moving parts are seen blurred. A LCD device where this disadvantage of the “contour blur of time-varying images” is relaxed is disclosed in the Patent Document 2 (the Japanese Non-Examined Patent Publication No. 2004-163829) published in 2004. This LCD device is shown in FIG. 2.
FIG. 2 is a functional block diagram showing the structure of the prior-art LCD device 150 disclosed in the Patent Document 2.
The prior-art LCD device 150 comprises a LCD panel 158 having data electrodes and scanning electrodes, an electrode driver section 151 for driving the data and scanning electrodes of the panel 158, a backlight unit 162 as a light source for irradiating light to the panel 158 from its back, and a light-source driver 161 for on-off driving the backlight unit 162 intermittently within a single vertical period.
An input image signal is delayed by the time corresponding to one frame period in a delayer section 156 and then, sent to a frame frequency converter section 152. The frame frequency converter section 152 converts the frame frequency of the input image signal thus delayed to a high frequency and outputs the input image signal thus converted to the electrode driver section 151. The electrode driver section 151 drives the data and scanning electrodes of the LCD panel 158 according to the input image signal thus received, thereby displaying images corresponding to the said input image.
A synchronizing signal extractor section 160 extracts the vertical synchronizing signal from the input image signal and supplies it to the light-source driver 161. The light-source driver 161 drives the backlight unit 162 based on the vertical synchronizing signal thus received, thereby turning on and off the backlight unit 162 within a single vertical period for the intermittent operation thereof.
A temperature detector section 153 detects the temperature of the inside of the LCD device 150. A frame memory (FM) 154 stores preceding frame data. A control CPU (Central Processing Unit) 155 detects the gradation transition between the current frame data and the preceding frame data which has been read from the frame memory 154, and outputs a predetermined control signal to the frame frequency converter section 152 based on the gradation transition of the above-described input image signal between the prior frame and the current frame and that between the current frame and the subsequent frame, and the temperature data detected by the temperature detector 153.
In response to the control signal outputted from the control CPU 155, the frame frequency converter section 152 converts the frame frequency of the input image signal to, for example, a higher frequency, thereby shortening the scanning period in one frame period to increase the liquid-crystal response period. For this reason, even if an image accompanying the gradation transition where the liquid-crystal response speed is low is inputted into the LCD device 150, the length of the liquid-crystal response period can be made satisfactory. As a result, the said image can be displayed after the liquid-crystal molecules have responded completely and the brightness of the emitted light has reached the target value thereof.
The backlight unit 162 has two types of lighting, i.e., the “global flash” type and the “scan” type. With the “global flash” type lighting, the backlight unit 162 is globally or entirely turned on and off. On the other hand, with the “scan” type lighting, the light-emitting surface of the backlight unit 162 is divided into a plurality of light-emitting regions in advance, and the light is sequentially emitted from the respective light-emitting regions in a predetermined order corresponding to the write scanning of the image signal.
With the prior-art LCD device 150 shown in FIG. 2, since the above-described structure is provided, not only the contour blur of time-varying images but also the afterimage thereof can be suppressed and therefore, high-quality time-varying images can be displayed, (See Abstract, FIGS. 1 to 3 and paragraphs 0032 to 0041 of the Patent Document 2.)
In addition, the Patent Document 2 discloses another structure that makes it possible to display high-quality time-varying images similar to that realized by the structure shown in FIG. 2 in the method where the write scanning for a black display signal (i.e., the reset scanning) is carried out subsequent to the write scanning of the image signal in one frame. With this structure, the image display period (i.e., the black display period) is controlled in response to the gradation transition of an image signal between the prior frame and the current frame and that between the current frame and the subsequent frame. Even if an image accompanying the gradation transition where the liquid-crystal response speed is low is inputted, the length of the liquid-crystal response period can be made satisfactory, and as a result, the said image can be displayed after the liquid-crystal molecules have responded completely and the brightness of the emitted light has reached the target one thereof. (See FIGS. 8 to 10 and paragraphs 0090 to 0098 of the Patent Document 2.)
To converge the emitted white light to a predetermined chromaticity within a short time while improving the quality of time-varying images in the above-described prior-art LCD device 150 explained with reference to FIG. 2, the structure of the above-described prior-art LCD device 100 shown in FIG. 1 may be used in combination. Concretely speaking, the structure of the LCD device 100 of FIG. 1 that the current quantity for the LEDs of each color is feedback-controlled based on the quantity of the received light of the chromaticity sensor 115 may be combined with the structure of the LCD device 150 of FIG. 2 that the backlight unit 162 is operated intermittently within one frame in such a way that white light having a predetermined chromaticity is emitted. In this case, it seems that the emitted white light may be converged to the predetermined chromaticity within a short time while the quality of time-varying images is improved.
However, if such the combination as above is adopted, the light-emitting period of each LED will be shortened. This is because the LEDs of the backlight unit 162 are operated intermittently in one frame (i.e., the LEDs are partitioned into n groups and operated sequentially) in the LCD device 150 of FIG. 2. Therefore, the quantities of the lights emitted from the respective LEDs the chromaticity sensor 115 receives, which performs the integration of the waveforms of the received lights and the outputting of the result of the integration, will reduce to a fraction of the number (a) of time division, i.e., (1/n), compared with the LCD device 100 where the scanning operation is not carried out.
As a result, there arise the above-described problem of the LCD device 100 of FIG. 1 that it takes a long time from the time immediately after the turn on to the time the emitted white light is converged to the predetermined chromaticity, and another problem that a large chromaticity error will occur when the convergence of the emitted lights is competed.